Aircraft: GOLDEN EAGLE 421
Phase of flight: Landing
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Ground Strike - Aircraft; Ground Event / Encounter Loss Of Aircraft Control

A Cessna 421C having undergone a complete and thorough Annual with overhauled/rebuilt engines; props; landing gear actuators and several other items was taken on a test flight for any issues resulting from the extensive disassembly and reassembly of parts and panels.The following are the facts:My Director of Maintenance (DOM) for the newly purchased Part 135 Air Charter and I had done most of the work on the aircraft during long days and nights trying to get it ready for an FAA inspection to be put back in service in March. As the owner of the air charter I called our Principal Operations Inspector (POI) at the local FSDO and let him/her know we were not going to meet the deadline in March and that we were going to wait until late April for the inspection; but nonetheless my DOM pushed to complete the remaining repairs by staying very late at night and into the wee morning hours; sometimes exceeding 16-hours of continuous work.My Director of Maintenance called on the Chief Pilot and the former Director of Operations to act as pilot and copilot on a short test run from North ZZZ1 to ZZZ2; and we both rode as passengers to observe aircraft gauges and look for general performance issues.At ZZZ2 we inspected the aircraft and noticed some minor hydraulic leaks from reservoir; engine oil leak from left engine; and some other minor issues that were not serious enough to be a problem for a return flight for further service. We filled all fuel tanks to the top to check for leaks.During run-up; brakes felt like they were not releasing well and a low hydraulic pressure light illuminated. Consensus among the three seasoned aviators was to free up the brakes by fiddling with the parking brake and hydraulic valve and fly back with the landing gear down the whole way in case the low hydraulic warning was in fact an indication of hydraulic issues; which may prevent the landing gears from dropping down for landing.My aviation repair experience is short and limited but I had the opportunity to read the chapter on human factors and the chain of events that generally lead to an incident/accident; and the importance of recognizing that and breaking that chain of events to prevent a catastrophe. However; hearing the three veterans of aviation conferring and deciding to continue to fly the aircraft; against my better judgment; I chose to remain silent and opted to be more vigilant of the aircraft gauges and look out the windows for signs of the hydraulic and engine oil leak from becoming too severe (as if that would be a lot of help at 9;000 feet).Even though the following incident didn't result from any of the issues noted above; the fact that the Pilot in Command (PIC) and the Second in Command (SIC) felt no need to be extra vigilant during the pre flight and/or to question the quality of workmanship during the Annual Inspection and ensuing repairs; in my opinion; was the 'missed opportunity' to break the chain of events that led to the incident below.Upon landing at the home ZZZ1 airfield; the front gear actuator broke just past the jam-nut; which had not been safety wired; as was the left gear. Both these actuators had been sent out to be rebuilt and reinstalled due to leaks; but not safety wired. The recently overhauled engines and props and additionally the nose of the aircraft were severely damaged; when the nose gear collapsed and the aircraft slid down the runway for over 1;200 feet. Fortunately there were no injuries to anyone onboard. Aircraft was in for Annual where it was discovered Nose Gear Retract Actuator and Left Gear Retract Actuator Leaking. Actuators were removed and sent out for overhaul. Overhauled Actuators were installed and rigged. Aircraft was connected to a hydraulic power unit while the aircraft was on jacks and the Landing Gear was cycled four times verifying landing gear lights operation and verifying no hydraulic leaks; and rigging and doors were correct per Cessna Maintenance Manual (CMM).Main Brake pads were changed at the same time as the actuators installation; four high speed taxi runs down the runway were accomplished to brake in the new pads per Cessna Maintenance Manuel. The next Day; in March 2015; the aircraft was flown to ZZZ2; no event landing all system operating normal. Return flight ZZZ2 to ZZZ1; no event until landing where Nose Gear collapsed upon landing. Nose Gear Retract Actuator broke at rod end. Reporter stated that he is a private pilot and that he and two other students who were in an aircraft technician school; were working under the direction of an Aircraft Maintenance Technician (AMT) who also had an Inspection Authority (IA) rating and was thought to be the Director of Maintenance for the recently purchased Part 135 Air Charter Cessna 421c aircraft they were working on. He did not install the Nose Landing Gear (NLG) actuator; but one of the other student's did. A bolt for the actuator rod end became difficult to install and started to strip; so the AMT told them to leave it loose and he would get to it later. But apparently he forgot and did not notice the actuator jam nut was not safety tied. The aircraft had flown at 140 knots with the gear down and the thinking was the vibration in flight caused the unsafety tied jam nut to back off causing the rod end to break on landing. A student mechanic and an Aircraft Maintenance Technician (AMT) report about a Nose Landing Gear (NLG) on a Cessna 421C aircraft that had collapsed on landing damaging both overhauled engines; props; and nose section. Nose Gear Retract Actuator had broken at the actuator rod end where a jam nut was found not safety tied. The aircraft had slid down the runway 1200 feet.